2. Propulsion: Axial Flow Compressor & Fan
Aerospace Engineering, International School of Engineering (ISE)
Academic year : (August – December, 2012)
Jeerasak Pitakarnnop , Ph.D.
October 27, 2012
Aircraft Propulsion

3. Introduction
First rotating component that the fluid encounters.
Basic function:
Impart kinetic energy to the working fluid by means of rotating blades, then
Convert the increase in energy to an increase in total pressure.
“The pressure ratio increases, the required fuel flow decreases and the extracted power increases”
October 27, 2012
Aircraft Propulsion

4. Introduction
Design of an efficient axial flow fan/compressor is a complex process which often involve success or failure of an engine  CFD tools can efficiently be use for complex 3D analysis and design.
Addition functions:
A small portion of the air is bled to provide some cockpit and electronic environmental control.
A small portion is bled to provide pressurized air for inlet anti-icing.
Some of the high-pressure “cool” air is directed to the turbine and used to reduce the temperature of hot turbine blades.
October 27, 2012
Aircraft Propulsion

5. Geometry Tip and housing diameter are approximately constant
through a compressor.
Rotor blades: do the work on fluid.
Stationary blades: do not input any energy but necessary for guiding the flow.
Typical number of stage is from 5 to 20, the total pressure ratio across a single stage is typically from 1.15 to 1.28.
October 27, 2012
Aircraft Propulsion

6. Geometry
Disk
October 27, 2012
Aircraft Propulsion

7. Inlet & Exit Guide Vanes
Inlet Guide Vanes (IGV):
Same function as stator vanes but the design is quite different.
They turn the incoming air, which is in the axial direction in a direction of 1st rotor blades incident free.
Exit Guide Vanes (EGV)
A set of stator vanes after the last stage that readies the flow for entrance to the combustor.
Design to add swirl to the flow, which aids in mixing within the combustor.
October 27, 2012
Aircraft Propulsion

8. Compressor Stage Operation
A stage:
a rotor wheel carrying blades
+ a stator assembly carrying stationary blades or vanes
Proper flow direction from IGV or stator of previous stage
Investigation of stage aerodynamics is usually carried out in a cascade tunnel, an experimental setup where single or multi-stage cascades are tested under simulated flow conditions.
Conceptual unwrapping
of middle section
Consider the flow at some midspan blade section (between hub and tip).
Absolute velocity V as seen by an external observer standing next to the engine.
Circumferential velocity U depending on rotational speed (rpm) and radial position.
Relative velocity Vrel as seen by an observer sitting on the rotating blade and moving with it.
3D effects occur in an actual compressor but for study purposes flow in a cascade is considered to be 2D.
October 27, 2012
Aircraft Propulsion

9. 2D Simulation in CFD
October 27, 2012
Aircraft Propulsion

10. Velocity Polygon or Triangles
October 27, 2012
Aircraft Propulsion

11. Inlet Guide Vanes
The axial flow velocity relative to the engine flame (absolute velocity) at the IGV inlet is c0.
The inlet flow to IGV is typically aligned with the axis of the engine
Exit blade angle relative to the axis of the engine..
The absolute flow velocity (velocity in the non rotating flame) at the IGV exit is c1.
Exit flow angle relative to the axis of the engine..
October 27, 2012
Aircraft Propulsion

12. 1st Stage Rotor
Relate the velocities in the stationary ref. frame to those in rotating frame
Resulting velocity in a rotating frame w1. has a flow angle β1.
If β1 = β’1 incidence angle is 0.
Difficult to happen in off-design conditions
The difference at tailing edge β2 and β’2 is called the deviation.
Abs. vel. c1 has component
in tangential direction cu1
In axial direction ca1
Rel. vel. w1 has component
in tangential direction wu1
In axial direction wa1
October 27, 2012
Aircraft Propulsion

13. 1st Stage Stator
The relative flow velocity (velocity in the rotating flame) at the rotor exit is w1.
The absolute flow velocity (velocity in the stationary flame) at the stator inlet is c1.
The absolute flow velocity (velocity in the stationary flame) at the stator exit is c2.
October 27, 2012
Aircraft Propulsion

14. 2nd Stage Rotor
October 27, 2012
Aircraft Propulsion

15. 2nd Stage Stator
October 27, 2012
Aircraft Propulsion

16. Side View of First Stage
Blade Height
Radius of the blade
October 27, 2012
Aircraft Propulsion

17. Single-Stage Energy Analysis
Relate the velocity from polygons to the pressure rise and other
Axial Flow Compressor component Trends
October 27, 2012
Aircraft Propulsion

18. Total Pressure Ratio
The equations is derived for a single stage (rotor and stator) using 2D planar mean line c.v. approach.
“Midway between hub and tip”
Power Input to the Shaft
Total Pressure Ratio of the Stage
Control Volume definition
for compressor stage
October 27, 2012
Aircraft Propulsion

19. Percent Reaction
A relation that approximates the relative loading of the rotor and stator based on the enthalpy rise:
October 27, 2012
Aircraft Propulsion

20. Incompressible Flow
For comparison, the pressure rise and percent reaction of a turbomachine with an incompressible fluid can be found from the following equations:
Power Input to the Shaft
Total Pressure Rise of the Stage
Percent Reaction
October 27, 2012
Aircraft Propulsion

21. Relationships of Velocity Polygons to Percent Reaction and Pressure Ratio
October 27, 2012
Aircraft Propulsion

22. Relationships of Velocity Polygons to Percent Reaction and Pressure Ratio
October 27, 2012
Aircraft Propulsion

23. Limit on Stage Pressure Ratio
The rotor is moving, the relative velocity must be used:
For the stator, which is stationary the relative velocity must be used:
1 and 2 refer to the stage inlet and midstage properties.
October 27, 2012
Aircraft Propulsion

24. Limit on Stage Pressure Ratio
Rotor
Stator
October 27, 2012
Aircraft Propulsion

25. Ex1 : Velocity Polygon
A stage approximating the size one of the last stages (rotor and stator) of a high-pressure compressor is to be analyzed. It rotates at 8000 rpm and compresses 127 kg s-1 of air. The inlet pressure and temperature are MPa and K, respectively. The average radius of the blades is mm and the inlet blade height is mm. The absolute inlet flow angle to the rotor is the same as the stator exit flow angle 15°, and the rotor flow turning angle is 25°. The stage has been designed so that the blade height varies and the axial velocity remains constant through it. The efficiency of stage is 90%. The value of cp and γ are kJ kg-1 K-1 and 1.361, respectively, which are based on the resulting value of T2 or average static temperature of the stage. The following details are to be found: blade heights at the rotor and stator exits, the static and total pressures at the rotor and stator exits, the required power for the stage, and the percent reaction for the stage.
October 27, 2012
Aircraft Propulsion